Originally posted by Tyler McAdams:
It has been noted that if you add a 33mhz/32bit card to a 64bit slot that the entire bus will slow to the 33mhz speed.
What has not been mentioned is if the 64bit/33mhz cards now operate in 32bit/33mhz compatibility mode or are the 64bit channel still active? In other words the still work at 64bit/33mhz instead of 64bit/66mhz or 32bit/33mhz.
Unless there is an extreme screw-up in the hardware or software, it
will use the 64 bit data paths. At the lowest level, the width of the
data transfer on a 64 bit PCI bus is controlled by two signals
(Req64 and Ack64) that are almost completely independent from the
rest of the protocol on the bus: a transaction will only be 64 bit
if the initiator activates the REQ64 line and the target acknowledges
this by activating the ACK64 signal.
The frequency, on the other hand, has to be the lowest common
denominator since a 33MHz board clocked at 66MHz is extremely
unlikely to fulfill the stricter timing requirements and to properly
understand what happens on the bus. It may for example believe that
it has been addressed when it was not the target of a transaction,
the result would not be pretty!
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